[KiXG-SHAw] SOl'ND THROUGH EARTH AND ROCK 81 



from the oscillator through the air were only just audible directly by 

 ear, the miscophone together with the amplifier was so sensitive as to 

 make it possible to hear these disturbances with great distinctness. 

 The microphone in this case was placed on a rock firmly embedded 

 in the soil and was thus situated only a few inches from the surface. 

 Further experiments in this direction showed that the sensitivity of 

 this arrangement to aerial sounds was remarkable: in circumstances 

 when, owing to atmospheric conditions, direction of wind, etc., the 

 oscillator could not be heard directly by ear, the vibrations could be 

 still be heard in the telephones. The sound of a voice under conditions 

 which made it just audible as a faint murmur at 150 yards could be 

 recognized in the microphone and understood. 



An important factor in this extreme degree of sensitiveness to 

 aerial sounds lies in the fact that the microphone receiver is placed 

 close to the ground. It is well known that even with a slight breeze 

 blowing (the atmosphere is rarely, if ever, completely at rest) the 

 motion of the air is one of large-scale eddy motion , except, possibly, 

 close to the surface of the earth where, owing to the pressure of obs- 

 tacles such as underbrush, long grass, etc., the air is comparatively 

 stagnant. As long as the dimensions of these obstacles are small 

 compared to a wave-length of sound it seems probable that their 

 effect in attenuating sound-waves is less marked than that of the 

 irregular motion of the atmosphere at a moderate distance from the 

 earth's surface. It is conceivable that the existence of a narrowly 

 confined region of good sound transmission close to the ground is 

 responsible for the extreme acuity to sound of small game, birds, 

 etc., as sound waves coming from a distant source would be destroyed 

 in travelling through a turbulent atmosphere a few feet above the 

 earth's surface long before being greatly attenuated in this relatively 

 stagnant ground layer. 



A few experiments were carried out with a microphone in the 

 neighbourhood of a trench being excavated for laying conduits. At 

 several points the bed rock had been uncovered and on one of these 

 ledges the "rock-microphone" was placed. Sound produced by the 

 blows of a pick were heard in the microphone very distinctly at a 

 distance of about 200 ft. through rock. There was no chance of 

 confusion at this short distance as to the medium of transmission 

 because it was observed that the sound in the rock was always succeeded 

 by the sound in the air-layer as well. The sound was so distinct that 

 it was reasonable to assume that if similar blows had been given to 

 that layer of rock at a distance many times as great, the sound could 

 have been detected in the microphone. The tramping of men could 

 also be heard, up to a distance of about 100 yards. 



